The semiconductor or integrated circuit (IC) industry aims to manufacture ICs with higher and higher densities of devices on a smaller chip area to achieve greater functionality and to reduce manufacturing costs. This desire for large scale integration has led to a continued shrinking of circuit dimensions and device features. The ability to reduce the size of structures, such as gate lengths in field-effect transistors and the width of conductive lines, is driven by lithographic performance.
With conventional lithography systems, radiation is provided through or reflected off a photomask or reticle to form an image on a semiconductor wafer. In the field of integrated circuits, a reticle can be classified as a plate that contains the pattern for one or more die but is not large enough to transfer a wafer-sized pattern all at once. A mask can be classified as a plate that contains a pattern large enough to pattern a whole wafer at a time. For the sake of brevity, the term “reticle” will be used herein to include both reticles and masks as defined above, as the various embodiments of the present invention apply to both. A reticle typically comprises a transparent substrate upon which is disposed an absorber material that typically comprises chromium but, alternatively, may comprise molybdenum and silicon, or other materials. The absorber material is covered by an antireflective coating (ARC) that typically comprises chromium oxide. Both the absorber material and the overlying ARC are patterned so that, during photolithography, a desired image is projected through the reticle onto the semiconductor wafer. Generally, the image is focused on the wafer to expose and pattern a layer of material, such as photoresist material. In turn, the photoresist material is utilized to define doping regions, deposition regions, etching regions, or other structures associated with ICs in one or more layers of the semiconductor wafer. The photoresist material can also define conductive lines or conductive pads associated with metal layers of an IC. Further, the photoresist material can define isolation regions, transistor gates, or other transistor structures and elements.
Older lithography systems are typically configured to expose the photoresist material at a radiation, conventionally produced by krypton fluoride (KrF) excimer lasers, having a wavelength of 248 nanometers (nm). However, because the resolution limit of features is, in part, dependent upon the exposure wavelength, it is desirable to pattern photoresist material using radiation at shorter exposure wavelengths (e.g., the wavelength range bounded approximately by, and including, 193 nm to 13.4 nm (193 nm, 157 nm, 126 nm, or 13.4 nm)). Unfortunately, reticles subjected to these shorter wavelengths may exhibit migration of material from the absorber material and/or the ARC to cover otherwise exposed areas of the transparent substrate. For example, over continued use, reticles with absorber materials that comprise chromium have been found to exhibit migration or spreading of material resulting in chromium and chromium oxide residue on the substrate about the mask features. Such a phenomenon also may be likely with other absorber materials. For example, because molybdenum and chromium have similar metallic structures, it may be expected that absorber material layers of molybdenum silicide will exhibit molybdenum and/or molybdenum oxide migration. The migrated material distorts the dimensions of the features of the mask, thus degrading the quality of the mask. While the migrated material may be removed from the mask by methods such as etching, this cleaning process increases costs of the photolithography process and results in down time unless an alternate reticle is available, which also increases costs of the process.
Accordingly, it is desirable to provide reticles for use in photolithography that do not exhibit material migration during exposure to ultraviolet radiation. It also is desirable to provide methods for fabricating reticles for use in photolithography that do not exhibit material migration during exposure to ultraviolet radiation. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention